Method for producing caoutchouc particles

ABSTRACT

The invention relates to a method for producing caoutchouc particles (K) by means of emulsion polymerisation in the presence of an emulsifier and a polymerisation initiator. Said particles contain A) 80 to 100 wt. % of one or more conjugated diene monomers (A) in relation to (K) and B) 0 to 20 wt. % of one or more monoethylenically unsaturated comonomers (B) in relation to (K) in the polymerised form. The inventive method is characterised in that 1) a mixture (M 1 ) containing water and an emulsifier is provided, 2) a mixture (M 2 ) containing one or more monomers in the monomer or polymerised form selected from styrole, α-methylstyrole, butadiene, n-butylacrylate, MMA and acrylnitrile and optionally comonomers is added, 3) polymerisation of the obtained mixture starts in the presence of a polymerisation initiator at temperature of 5 to 95° C., 4) a mixture (M 3 ) containing 0 to 100 wt. % of the comonomers (B) in relation to (B) and 0 to 25 wt. % of the diene monomers (A) in relation to (A) is added, 5) a mixture (M 4 ) containing the (remaining) diene monomers (A) and the (remaining) comonomers (B) is proportioned and polymerised and 6) polymerisation is terminated when there is conversion of more than 90 and less than 95% in relation to the sum of the monomers.

The invention relates to a process for preparing rubber particles Kcomprising in polymerized form

A) from 80 to 100% by weight, based on K, of one or more conjugateddiene monomers A, and

B) from 0 to 20% by weight, based on K, of one or more monoethylenicallyunsaturated comonomers B

by emulsion polymerization in the presence of an emulsifier and apolymerization initiator. The invention further relates to the rubberparticles K prepared by the process and to their use as a constituent ofthermoplastic molding compounds, dispersions, paper coating compositionsand surface coatings, and for textile finishing.

Processes for the emulsion polymerization of rubbers based on butadieneor other rubber-forming dienes where some or all of the monomers aremetered in during the polymerization (known as feed techniques) areknown.

For instance, DE-A 34 47 585 describes styrene-butadiene latices whosegraft base contains at least 86% by weight styrene and whose graftcontains at least 62% by weight styrene, which are prepared by a feedtechnique with or without a polystyrene seed latex. This document doesnot disclose butadiene-rich latices.

EP-A 387 855 discloses a feed technique for preparing polymer particlescomprising, inter alia, styrene and butadiene where a seed latex havinga weight-average molecular mass {overscore (M)}_(w) of only from 500 to10,000 is used. Although the comparative examples do include seedlatices of relatively high molecular mass, the butadiene fraction in thefeed stream is not more than 40% by weight, i.e., these polymerparticles are also low in butadiene.

EP-A 814 103 discloses a feed technique for preparing polymerdispersions comprising, inter alia, styrene and butadiene where a seedlatex is mixed with the monomers and emulsified and this emulsion isthen metered into the heated polymerization reactor. The seed latex,accordingly, is not included in the initial charge but instead ispresent in the feed stream, leading to an undesirably wide particle sizedistribution.

EP-A 276 894 discloses adhesive compositions comprising starch andstyrene-butadiene latices. The latices are prepared by a feed techniqueusing a polystyrene seed latex and contain more than 60% by weightstyrene, and are therefore low in butadiene.

EP-A 792 891 discloses a process for preparing latices based onconjugated dienes such as butadiene, where a seed latex, comprising, inparticular, polystyrene particles of from 10 to 80 nm in diameter, isincluded in the initial charge. In the presence of an activator (i.e.initiator) and an emulsifier, the entirety of the monomers are meteredin such that a defined relationship between polymerization rate andmonomer addition rate is established. The polymerization is only endedwhen the conversion is at least 95%. Accordingly, only the seed latex isincluded in the initial charge, and not a portion of the monomers. Incomparative experiments, all monomers are introduced in the initialcharge together with the seed latex (batch, i.e., no feed stream).

EP-A 761 693 discloses a process for preparing diene latices where acertain fraction of the reaction mixture is included in the initialcharge, so that a defined vessel filling level is achieved, and theremainder of the reaction mixture is run in under controlled conditions.Using this process it is possible to produce only small particles offrom 60 to 120 nm in diameter. The increased utilization of the gasspace in the vessel for cooling purposes, which is intrinsic to theprocess, leads to increased formation of coagulum. Moreover, thepolymerization time is uneconomically long.

The latices and polymer particles of the prior art have the followingdisadvantages: they are low in butadiene, or long polymerization timesare needed in order to prepare relatively large particles. Moreover, theprior art processes have large amounts of unreacted monomers during thepolymerization reaction (known as hold-up), which may be a safety riskin the case of disruptions (poor operational safety).

It is an object of the present invention to provide a process which doesnot have the disadvantages depicted. In particular, the intention was toprovide a process which enables diene-rich rubber particles having dienecontents ≧80% by weight and particle sizes above 100 nm to be preparedin a short time. Furthermore, the process is to be operationally safe byvirtue of the fact that the proportion of unreacted monomers (hold-up)in the reactor is kept low.

We have found that this object is achieved by the process defined at theoutset. This process comprises

1) initially introducing a mixture M1 comprising, based on M1,

M1 a) from 20 to 100% by weight of the water needed to prepare theemulsion (emulsion water), and

M1 b) from 0.1 to 100% by weight of the emulsifier,

2) simultaneously or subsequently adding a mixture M2 comprising, basedon M2,

M2 a) from 90 to 100% by weight of one or more monomers selected fromstyrene, α-methylstyrene, butadiene, n-butyl acrylate, methylmethacrylate and acrylonitrile, and

M2 b) from 0 to 10% by weight of one or more copolymerizable monomers,

the monomers M2 a) and M2 b) being added alternatively in polymerizedform as a seed latex, or in monomeric form with subsequent in situpolymerization to give a seed latex, or as a mixture of polymerized andmonomeric form, and the seed latex polymer having a weight-averagemolecular mass of more than 20,000,

3) then starting the polymerization of the resulting mixture in thepresence of the polymerization initiator at temperatures of from 5 to95° C.,

4) simultaneously or subsequently adding a mixture M3 comprising

M3 a) from 0 to 100% by weight, based on B, of the comonomers B, and

M3 b) from 0 to 25% by weight, based on A, of the diene monomers A,

5) simultaneously or subsequently metering in a mixture M4 comprising

M4 a) the remaining 75 to 100% by weight, based on A, of the dienemonomers A, and

M4 b) the remaining 0 to 100% by weight, based on B, of the comonomersB,

and carrying out polymerization, and

6) subsequently ending the polymerization at a conversion above 90 andbelow 95%, based on the sum of the monomers,

the remaining 0 to 80% by weight of the emulsion water and the remaining0 to 99.9% by weight of the emulsifier being added individually orseparately from one another in one or more of steps 2) to 5).

We have additionally found the rubber particles K prepared by theprocess and their uses as specified at the outset.

The rubber particles K prepared by the process of the invention comprisein polymerized form

A) from 80 to 100, preferably from 85 to 100% by weight, based on K, ofone or more conjugated diene monomers A, and

B) from 0 to 20, preferably from 0 to 15% by weight, based on K, of oneor more monoethylenically unsaturated comonomers B which arecopolymerizable with the diene monomers A to give copolymers.

Suitable diene monomers A are all dienes having conjugated double bonds,especially butadiene, isoprene, chloroprene or mixtures thereof.Particular preference is given to butadiene or isoprene or mixturesthereof. With very particular preference, butadiene is used.

Suitable comonomers B are all monoethylenically unsaturated monomers,especially

vinylaromatic monomers such as styrene, styrene derivatives of theformula I

where R¹ and R² are hydrogen or C₁-C₈-alkyl and n is 0, 1, 2 or 3;

methacrylonitrile, acrylonitrile;

acrylic acid, methacrylic acid, and also dicarboxylic acids such asmaleic acid and fumaric acid and also their anhydrides such as maleicanhydride;

nitrogen-functional monomers such as dimethylaminoethyl acrylate,diethylaminoethyl acrylate, vinylimidazole, vinylpyrrolidone,vinylcaprolactam, vinylcarbazole, vinylaniline, acrylamide;

C₁-C₁₀ alkyl esters of acrylic acid, such as methyl acrylate, ethylacrylate, n-propyl acrylate, i-propyl acrylate, n-butyl acrylate,isobutyl acrylate, sec-butyl acrylate, tert-butyl acrylate, ethylhexylacrylate, and the corresponding C₁-C₁₀ alkyl esters of methacrylic acid,and also hydroxyethyl acrylate;

aromatic and araliphatic esters of acrylic acid and methacrylic acidsuch as phenyl acrylate, phenyl methacrylate, benzyl acrylate, benzylmethacrylate, 2-phenylethyl acrylate, 2-phenylethyl methacrylate,2-phenoxyethyl acrylate and 2-phenoxyethyl methacrylate;

N-substituted maleimides such as N-methyl-, N-phenyl- andN-cyclohexylmaleimide;

unsaturated ethers such as vinyl methyl ether;

crosslinking monomers, as described later on below in connection withthe comonomers M2 b);

and mixtures of these monomers.

Preference is given to styrene, α-methylstyrene, p-methylstyrene,t-butylstyrene, n-butyl acrylate, methyl methacrylate (MMA),acrylonitrile or mixtures thereof as comonomers B. Particular preferenceis given to styrene, n-butyl acrylate, methyl methacrylate,acrylonitrile or mixtures thereof. In particular, styrene is used.

In one particular embodiment the rubber particles K are prepared using,based on K,

A) from 70 to 99.9, preferably from 90 to 99% by weight of butadiene,and

B) from 0.1 to 30, preferably from 1 to 10% by weight of styrene,acrylonitrile, MMA, n-butyl acrylate or mixtures thereof.

The rubber particles K are prepared by the technique of emulsionpolymerization. Customary emulsifiers are used, examples being alkalimetal salts of alkyl- or alkylarylsulfonic acids, alkyl sulfates, fattyalcohol sulfonates, salts of higher fatty acids with 10 to 30 carbonatoms, sulfosuccinates, ether sulfonates or resin soaps. It is preferredto employ the alkali metal salts, especially the Na and K salts, ofalkylsulfonates or fatty acids with 10 to 18 carbon atoms. Preferredemulsifiers used are the abovementioned salts of higher fatty acids.Potassium and sodium stearate are particularly preferred. It is alsopossible to use mixtures of different emulsifiers.

In general, the emulsifiers are used in amounts of from 0.1 to 5% byweight, in particular from 0.2 to 3% by weight, based on the monomers Aand B used. Amounts of from 0.2 to 1% by weight are particularlypreferred.

The following comments apply to the polymerization initiators: forstarting the polymerization reaction, suitable free-radical initiatorsare all those which decompose at the chosen reaction temperature, i.e.,both those which decompose by heat alone and those which do so in thepresence of a redox system. Suitable polymerization initiators arepreferably free-radical initiators, examples being peroxides such aspreferably peroxosulfates (for instance sodium or, with particularpreference, potassium persulfate) and azo compounds such asazodiisobutyronitrile. It is also possible, however, to use redoxsystems, especially those based on hydroperoxides such as cumenehydroperoxide. Mixtures of different initiators may also be used.

In general, the polymerization initiators are used in an amount of from0.1 to 1% by weight, based on the monomers A and B.

The dispersion is preferably prepared using sufficient water for thefinished dispersion to have a solids content of from 30 to 60,preferably from 40 to 55% by weight. It is common to operate at awater/monomer ratio of from 2:1 to 0.7:1.

The process of the invention comprises six steps.

In step 1), a mixture M1 is introduced. It comprises, based on M1,

M1 a) from 20 to 100, preferably from 50 to 100% by weight of the waterneeded to prepare the emulsion (emulsion water), and

M1 b) from 0.1 to 100, preferably from 15 to 100% by weight of theemulsifier.

The mixture M1, accordingly, may contain the entirety or only part ofthe water needed to prepare the emulsion (referred to hereinbelow asemulsion water). The same applies, mutatis mutandis, to the emulsifier.

In step 2), simultaneously with step 1) or subsequently, a mixture M2 isadded. It contains, based on M2,

M2 a) from 90 to 100, preferably from 95 to 100% by weight of one ormore monomers selected from styrene, α-methylstyrene, butadiene, n-butylacrylate, MMA and acrylonitrile, and

M2 b) from 0 to 10, preferably from 0 to 5% by weight of one or morecopolymerizable monomers.

Suitable comonomers M2 b) are all monomers already stated for thecomonomers B, plus crosslinking monomers.

Crosslinking monomers are bifunctional or polyfunctional comonomershaving at least two olefinic nonconjugated double bonds, examples beingdivinyl esters of dicarboxylic acids such as of succinic acid and adipicacid, diallyl and divinyl ethers of bifunctional alcohols such as ofethylene glycol and of 1,4-butanediol, diesters of acrylic acid andmethacrylic acid with said bifunctional alcohols, 1,4-divinylbenzene,and triallyl cyanurate. Particular preference is given to the acrylicester of tricyclodecenyl alcohol, which is known under the namedihydrodicyclopentadienyl acrylate, and to the allyl esters of acrylicand methacrylic acid.

If crosslinking monomers are used as comonomers M2 b), their proportionis preferably from 0.01 to 5, in particular from 0.05 to 3% by weight,based on M2.

The monomers M2 a) and M2 b) may be added either in polymerized form,i.e., as a seed latex comprising polymer particles plus water M2 c), orin monomeric form. If they are added in monomeric form, they arepolymerized in situ in step 3) (starting of the polymerization) to givea seed latex, i.e., in this case the seed latex is formed in situ beforeor during the polymerization reaction of the diene monomers A and thecomonomers B, or, before or during the polymerization reaction of step3), particles are formed which serve as growth nuclei for the emulsionpolymerization.

The monomers M2 a) and M2 b) may also be added as a mixture ofpolymerized and monomeric form, i.e., a portion of M2 a) and M2 b) hasalready been incorporated in the polymerized seed latex and theremaining portion is polymerized in situ as described.

The polymers of step 2) are composed preferably of from 95 to 100% byweight of styrene or n-butyl acrylate and from 0 to 5% by weight of theaforementioned crosslinking monomers (i.e., a seed latex comprisingcrosslinked or uncrosslinked polystyrene or crosslinked or uncrosslinkedpoly-n-butyl acrylate). Polystyrene polymer particles are particularlypreferred.

In accordance with the invention, the seed latex particles have aweight-average molecular mass {overscore (M)}_(w) of more than 20,000,as determined by GPC measurement. The seed latex, accordingly, containsno low molecular mass polymers (weight average), but only polymerparticles of customary molecular weight.

Preferably, the seed latex particles have a weight-average particle sized₅₀ of from 10 to 100, with particular preference from 10 to 80, inparticular from 20 to 70 nm.

Thereafter, in step 3) of the process of the invention, thepolymerization of the mixture obtained in steps 1) and 2) is started inthe presence of the polymerization initiator at temperatures of from 5to 95° C., preferably from 40 to 95° C.

The remaining 0 to 80% by weight of the emulsion water and the remaining0 to 99.9% by weight of the emulsifier are added individually orseparately from one another in one or more of the steps 2) to 5).

The polymerization initiators—already described above—are added to thereaction mixture, for example, discontinuously in the form of the totalamount at the beginning of the reaction, or, divided into two or moreportions, at the beginning and at one or more subsequent points in time,or continuously throughout a certain time interval. The continuousaddition may also be made along a gradient which may, for example, beascending or descending, linear or exponential, or else stepwise (stepfunction).

In particular, the initiators may be added as early as in step 1) of theprocess or not until during step 2) or 3) or 4) or 5). Similarly, aportion of the initiators may be added in step 1) and a further portionin step 2) and/or step 3) and/or 4) and/or 5). Preferably, one portionof the initiator is added in step 2) and the remaining portion meteredin in step 3) of the process.

In one preferred embodiment, the initiator is added in step 1) and/or 3)and further initiator is added in step 4) and/or 5) in the form of ametering program (defined metering).

In another preferred embodiment, initiator is added in step 3) andfurther initiator is metered in subsequently during steps 4) and 5) instages or continuously or in intervals.

The same applies, mutatis mutandis, to the mode of addition of theemulsifiers. Preferably, one portion of the emulsifiers is included inthe initial charge and one portion is run in with the remaining emulsionwater.

Moreover, it is possible to use molecular weight regulators such as, forexample, ethylhexyl thioglycolate, n- or t-dodecyl mercaptan or othermercaptans, terpinols, and dimeric α-methylstyrene or other compoundssuitable for regulating the molecular weight. The molecular weightregulators are added discontinuously or continuously to the reactionmixture, as described above for the initiators and emulsifiers.

To maintain a constant pH of preferably from 7 to 11, more preferablyfrom 8 to 10, it is possible to use buffer substances such asNa₂HPO₄/NaH₂PO₄, sodium hydrogen carbonate/sodium carbonate, or buffersbased on citric acid/citrate. Other buffer systems are also suitable,provided they keep the pH within the stated range. Regulators and buffersubstances are used in the customary amounts, rendering further detailsof this superfluous.

In step 4), simultaneously with step 3) or subsequently, a mixture M3 isadded. It contains

M3 a) from 0 to 100, preferably from 50 to 100 and with particularpreference from 95 to 100% by weight, based on B, of the comonomers B,and

M3 b) from 0 to 25, preferably from 0 to 20% by weight, based on A, ofthe diene monomers A.

Preferably, the entirety of the comonomers B is metered in in step 4).

In step 5), simultaneously with step 4) or subsequently, a mixture M4 ismetered in, containing

M4 a) the remaining 75 to 100, preferably from 80 to 100% by weight,based on A, of the diene monomers A, and

M4 b) the remaining 0 to 100, preferably from 0 to 50 and withparticular preference from 0 to 5% by weight, based on B, of thecomonomers B,

and the monomers are polymerized.

As already mentioned, initiators and emulsifiers may also be metered inin step 5). This is preferred. The monomers A and, if appropriate, B,the initiators and emulsifiers may be metered in together as a mixture(a feed stream) or—preferably—separately from one another as two or morefeed streams. Molecular weight regulators as well are metered inpreferably as a feed stream, either alone or in a mixture with themetered monomers and/or emulsifiers. In one preferred embodiment, onefeed stream comprises the monomers, another feed stream comprises theinitiators, and a further feed stream comprises the emulsifiers.

In another preferred embodiment, the monomers A and, if appropriate, B,the emulsifiers and, if appropriate, the regulators are emulsifiedtogether with a portion of the emulsion water and are metered in in theform of this emulsion. This emulsion may be prepared, for example, in aseparate vessel, or else may be prepared continuously by means of activeor passive emulsifying systems, examples being inline mixers, Sulzermixers, or dispersers operating in accordance with a rotor-statorprinciple (e.g., Dispax®, IKA).

In another, particularly preferred embodiment, one feed stream comprisesa mixture of monomers, water, molecular weight regulator and emulsifiersand a further feed stream comprises the initiators. In a particularlypreferred embodiment more than 50% by weight in each case

of the total amount of the initiators,

of the total amount of the emulsifiers, and

of the total amount of the molecular weight regulators are not includedin the initial charge in step 1) but instead are metered in in step 5).

The metered addition may take place at a constant rate or along agradient, which may, for example, be ascending or descending, linear,exponential, or stepwise (step function). This applies, for example, tomonomers, emulsifiers, initiators and/or regulators.

In particular, in the case of more than one feed stream, the respectivemetered additions may differ in their duration. For example, the feedstream of monomers and/or emulsifiers may be metered in over a shortertime than the feed stream of the initiators. This is preferred.

Preferably, the duration of the metered addition of the monomers A and,if appropriate, B in step 5) is from 1 to 18, with particular preferencefrom 2 to 16 and in particular from 4 to 14 hours.

In one preferred embodiment, the rate of metered addition of themonomers A and B in step 5) is chosen so that at no point in time duringthe polymerization reaction of A and B are more than 50% by weight ofthe entirety of monomeric A+B present in the polymerization reactor.

In one particularly preferred embodiment, the rate of metered additionof the monomers A and B in step 5) is chosen so that up to a conversionof 50%, based on the sum of A+B, less than 30%, based on the sum ofunpolymerized and polymerized monomers A+B, of unpolymerized monomersare present in the polymerization reactor.

In a preferred embodiment of the process, the polymerization conditions,especially the nature, the amount and the metering (mode of addition) ofthe emulsifier, are chosen in a manner known per se such that theweight-average particle size d₅₀ of the rubber particles K is from 80 to800, preferably from 100 to 400, with particular preference from 110 to350 and in particular from 120 to 300 nm.

In the subsequent step 6) of the process, the polymerization is ended ata conversion of more than 90 and less than 95%, based on the sum of themonomers. In other words, the polymerization is ended when the followingcondition applies to the conversion U: 90%<U<95%.

The polymerization is ended in a customary manner, for example, bylowering the reactor temperature or by adding inhibiting substances,such as diethylhydroxylamine, for example, or by removing the unreactedmonomers, for instance, by letting down the reactor, or by a combinationof these termination methods.

It is preferred to establish a temperature gradient during theindividual steps of the process, e.g.,

step 4): starting at from 60 to 80° C.,

step 5): during the polymerization, a gradient up to a maximumtemperature of from 75 to 95° C.,

step 6): ending by cooling to 50° C. and/or removing the monomers.

Heating and cooling may be carried out, for example, linearly orstepwise, in accordance with another function. In particular, theheating may follow the adiabatic course of the reaction, utilizing theheat of reaction.

Normally, the polymerization of the rubber particles K is conductedunder reaction conditions chosen so as to give rubber particles having adefined state of crosslinking. Examples of parameters which areessential to this are the reaction temperature and reaction time, theratio of monomers, regulator, initiator and the feed rate, and theamount of and timing of the addition of regulator and initiator, andalso the nature and amount of any crosslinking monomers used.

One method of characterizing the state of crosslinking of crosslinkedpolymer particles is the measurement of the swelling index SI, which isa measure of the swellability of a more or less strongly crosslinkedpolymer by a solvent. Examples of customary swelling agents are methylethyl ketone and toluene. The SI of the rubber particles of theinvention is usually in the range from 10 to 60, preferably from 15 to55 and with particular preference from 18 to 50.

Another method of characterizing the state of crosslinking is themeasurement of NMR relaxation times of mobile protons, known as the T₂times. The higher the degree of crosslinking of a particular network,the lower its T₂ times. Customary T₂ times for the rubber particles ofthe invention are in the range from 2.0 to 4.5 ms, preferably from 2.5to 4.0 ms and with particular preference from 2.5 to 3.8 ms, measured onfilmed samples at 80° C.

A further measure to characterize the rubber particles and their stateof crosslinking is the gel content, i.e., that product fraction which iscrosslinked and is therefore insoluble in a certain solvent. It issensible to determine the gel content in the same solvent as theswelling index. For the rubber particles of the invention, customary gelcontents are in the range from 45 to 90%, preferably from 50 to 85% andwith particular preference from 55 to 80%.

The swelling index is determined, for example, by the following method:approximately 0.2 g of the solid of a rubber particle dispersion filmedby evaporating the water is swollen in a sufficient quantity (e.g., 50g) of toluene. After, say, 24 h, the toluene is filtered off withsuction and the sample is weighed. The sample is then dried underreduced pressure and weighed again. The swelling index is the ratio ofthe weight after the swelling process to the dry weight after the seconddrying operation. Accordingly, the gel content is calculated from theratio of the dry weight after the swelling step to the initial weightbefore the swelling step (×100%).

The T₂ time is determined by measuring the NMR relaxation of a rubberparticle dispersion sample from which the water has been removed toleave a film. For this purpose, for example, the sample is dried in airovernight and at 60° C. for 3 h under reduced pressure and then measuredwith an appropriate measuring instrument, an example being the minispecinstrument from Bruker, at 80° C. It is only possible to-compare samplesmeasured by the same method, since the relaxation is highlytemperature-dependent.

The rubber particles K may be used as they are or else further monomersmay be polymerized on, especially by grafting.

These monomers which are polymerized on generally form a shell whichencloses the rubber particles K. In the case of a graft polymerization,it is referred to as the graft shell or graft, and graft polymers areformed.

The shell may be composed of all monomers which may be free-radicallypolymerized in emulsion.

The preparation of the shell may take place under the same conditions asfor the preparation of the rubber particles K, it being possible toprepare the shell in one or more process steps (single-stage ormultistage grafting). It is also possible to polymerize on more than twoshells, by changing the monomers appropriately. The transitions betweenthe shells (stages) may be sharply defined or tapered. Further detailson the preparation of the graft polymers are described in DE-A 12 60 135and 31 49 358.

After the end of the polymerization—whether with or without a shellpolymerized on—the rubber particles K are present as a dispersion inwater. This dispersion may either be processed further as it is or elsethe rubber particles K may be separated from the aqueous phase. Thisoperation takes place in a manner known per se, for example, by sieving,filtering, decanting or centrifuging, it being possible to dry therubber particles further, if required, in a customary manner, forinstance, by means of hot air, spray drying, or using a pneumatic dryer.

The rubber particles K prepared by the process of the invention may beput to diverse uses. Merely by way of example, mention may be made oftheir use in unprocessed form as a dispersion for paints (emulsionpaints) or in papermaking (paper coating compositions) and also assurface coatings and for textile finishing. Similarly, they may be usedas a graft base for impact modification of thermoplastic moldingcompounds.

The production of thermoplastic molding compounds comprising the rubberparticles K may take place by conventional methods, for example, byincorporating the still moist or dried rubber particles K into thethermoplastic matrix at above the melting point of the matrix, inparticular at temperatures of from 150 to 350° C., in customary mixingapparatus, such as extruders or compounders. It is also possible toincorporate the dispersion of the rubber particles K as it is directlyinto the thermoplastics, with the dispersion water being removed in thecourse of incorporation in a customary manner, for instance, as steam byway of devolatilization equipment.

The stated average particle size d comprises the weight average of theparticle size, as determined using an analytical ultracentrifuge inaccordance with the method of W. Scholtan and H. Lange, Colloid-Z. undZ.- Polymere 250 (1972) 782 to 796. The ultracentrifuge measurementprovides the integral mass distribution of the particle diameter of asample. From this it is possible to derive the percentage by weight ofthe particles having a diameter equal to or smaller than a certain size.

The d₁₀ value indicates that particle diameter at which 10% by weight ofall particles have a smaller diameter and 90% by weight have a largerdiameter. Conversely, the d₉₀ value is that at which 90% by weight ofall particles have a smaller diameter and 10% by weight have a largerdiameter than the diameter corresponding to the d₉₀ value. Theweight-average particle diameter d₅₀ indicates that particle diameter atwhich 50% by weight of all particles have a larger diameter and 50% byweight have a smaller particle diameter. The d₁₀, d₅₀ and d₉₀ valuescharacterize the breadth Q of the particle size distribution, such thatQ=(d₉₀-d₁₀)/d₅₀. The smaller Q is, the narrower the distribution.

EXAMPLES

The fraction of monomers present in the reactor which have not yetreacted at a certain conversion is referred to hereinbelow as theholdup. It is based on the total amount of monomers added to the reactorup until the time in question.

Deionized water was used.

The polystyrene seed latex used had an average molecular mass of thepolystyrene particles of more than 100,000 as measured by GPC.

Example 1

In a steel autoclave (120 l) containing 50 kg of water, 0.189 kg ofpotassium stearate, 150 g of sodium hydrogen carbonate (NaHCO₃) and 83 gof potassium persulfate (KPS) were dissolved at 63° C. The temperaturewas then increased to 67° C. Over the course of 35 minutes a mixture of8.658 kg of butadiene and 3.367 kg of styrene was metered into theinitial charge. 10 minutes after the start of the metered addition, 160g of tert-dodecyl mercaptan (TDM) were metered into the vessel.

During the subsequent phases of the polymerization, the temperature wasincreased in steps to the final temperature. 36.8 kg of butadiene weremetered into the reactor over the course of 10 h (feed stream 2),beginning 2 hours after the start of the first metered addition. 6 hafter the start of feed stream 2, a further 160 g of TDM were introducedinto the reactor. One hour after the end of feed stream 2, a further 160g of TDM were added. After a further 2 h, the reaction mixture washeated to the final temperature of 75° C. After a further 4 h, thereaction was terminated by cooling to 50° C. and letting down theautoclave.

The conversion was 93.8% at a solids content of 46.9% after a total timeof 20 h; the particle size d₅₀ was 143 nm and the SI was 33. The holdupat 50% conversion was approximately 25%.

Example 2

In a steel autoclave (120 l) containing 48.5 kg of water, 0.189 kg ofpotassium stearate, 150 g of NaHCO₃ and 14 g of KPS were dissolved at63° C. The temperature was then increased to 70° C. Over the course of35 minutes a mixture of 8.658 kg of butadiene and 3.367 kg of styrenewas metered into the initial charge. 10 minutes after the start of themetered addition, 160 g of TDM were metered into the vessel.

The temperature was raised to 75° C. and 36.03 kg of butadiene weremetered into the reactor over the course of 9 h (feed stream 2),beginning 2 hours after the start of the first metered addition. 6 hafter the start of feed stream 2, a further 160 g of TDM were introducedinto the reactor. Directly after the end of feed stream 2, a further 160g of TDM were added. After a further 4 h, the reaction was terminated bycooling to 50° C. and letting down the autoclave.

The conversion was 91.4% at a solids content of 44.8% after a total timeof 16 h; the particle size d₅₀ was 153 nm and the SI was 29. The holdupat 50% conversion was approximately 20%.

Example 3

In a steel autoclave (120 l) containing 46.3 kg of water, 0.150 kg ofpotassium stearate, 150 g of NaHCO₃ and 14 g of KPS were dissolved at70° C. Then 291 g of a polystyrene seed latex having a particle size of29 nm and a solids content of 33% were added. Over the course of 2.5 h,a mixture of 8.637 kg of butadiene and 3.359 kg of styrene and 120 g ofTDM were metered into the initial charge. After the end of the feedstream, the temperature was raised to 75° C.

30 minutes after the end of the first feed stream, 35.987 kg ofbutadiene and 360 g of TDM were metered into the reactor over the courseof 9.5 h (feed stream 2). A mixture of 130 g of KPS and 4500 g of waterwere metered into the reactor over the course of 8 h, beginning 1 hourafter the start of feed stream 2. 3 h after the end of feed stream 2,the reaction was terminated by cooling to 50° C. and letting down theautoclave.

The conversion was 93% for a solids content of 45.6% after a total timeof 16 h; the particle size d₅₀ was 134 nm and the SI was 23. The holdupat 50% conversion was approximately 10%.

Example 4

In a steel autoclave (120 l) containing 30.61 kg of water, 0.050 kg ofpotassium stearate, 257 g of NaHCO₃ and 14 g of KPS were dissolved at70° C. Then 218 g of a polystyrene seed latex having a particle size of29 nm and a solids content of 33% were added. Over the course of 35minutes, 3.353 kg of styrene were metered into the initial charge.

During the subsequent phases of the polymerization, the temperature wasincreased in steps to the final temperature. An emulsion comprising 100g of potassium stearate, 15.745 kg of water, 44.546 kg of butadiene and479 g of TDM was metered into the reactor over the course of 9.5 h (feedstream 2), beginning 25 minutes after the end of the first meteredaddition. A mixture of 130 g of KPS and 4500 g of water was metered intothe reactor over the course of 10.5 h, commencing simultaneously withfeed stream 2. 7.5 h after the start of feed stream 2, the reactionmixture was heated to the final temperature of 80° C. 6 h after the endof feed stream 2, the reaction was terminated by cooling to 50° C. andletting down the autoclave.

The conversion was 94.4% at a solids content of 46.5% after a total timeof 17 h; the particle size (d₅₀) was 175 nm and the SI was 30. Theholdup at 50% conversion was approximately 10%.

Example 5

In a steel autoclave (120 l) containing 30.6 kg of water, 0.050 kg ofpotassium stearate, 254 g of NaHCO₃ and 14 g of KPS were dissolved at70° C. Then 218 g of a polystyrene seed latex having a particle size of29 nm and a solids content of 33% were added. Over the course of 2.5 h,a mixture of 8.62 kg of butadiene, 3.35 kg of styrene and 120 g of TDMwas metered into the initial charge. A mixture of 4500 g of water and129 g of KPS was run in over the course of 12.5 h, commencing 1 h afterthe start of the first metered addition.

After the end of the addition of the above monomers, during thesubsequent phases of the polymerization, the temperature was raised insteps to the final temperature. 30 minutes after the end of the firstfeed stream, an emulsion comprising 35.924 kg of butadiene, 100 g ofpotassium stearate, 15.745 kg of water and 360 g of TDM were meteredinto the reactor over the course of 9.5 h (feed stream 2). In the courseof feed stream 2, the internal reactor temperature was increased insteps until reaching a final temperature of 80° C. 7.5 h after the startof feed stream 2. 6 h after the end of feed stream 2, the reaction wasterminated by cooling to 50° C. and letting down the autoclave.

The conversion was 92% for a solids content of 45.2% after a total timeof 19 h; the particle size d₅₀ was 204 nm and the SI was 46. The holdupat 50% conversion was approximately 20%.

Example 6

In a steel autoclave (120 l) containing 30.6 kg of water, 0.015 kg ofpotassium stearate, 257 g of NaHCO₃ and 29 g of KPS were dissolved at70° C. Then 218 g of a polystyrene seed latex having a particle size of29 nm and a solids content of 33% were added. 3.35 kg of styrene weremetered into the initial charge over the course of 35 minutes.

25 minutes after the end of the first feed stream, an emulsioncomprising 44.52 kg of butadiene, 15.745 kg of water, 135 g of potassiumstearate and 479 g of TDM was metered into the reactor over the courseof 9.5 h (feed stream 2). In the course of feed stream 2, thetemperature was raised in steps until, directly after the end of feedstream 2, a final temperature of 80° C. was reached. Beginning at thesame time, a mixture of 144 g of KPS and 4500 g of water was meteredinto the reactor over the course of 15 h. 12 h after the end of feedstream 2, the reaction was terminated by cooling to 50° C. and lettingdown the autoclave.

The conversion was 91% for a solids content of 44.6% after a total timeof 22.5 h; the particle size d₅₀ was 251 nm and the SI was 43. Theholdup at 50% conversion was approximately 29%.

We claim:
 1. A process for preparing rubber particles K comprising inpolymerized form A) from 80 to 100% by weight, based on K, of butadieneor isoprene or mixtures thereof (diene monomers A), and B) from 0 to 20%by weight, based on K, of one or more monoethylenically unsaturatedcomonomers B by emulsion polymerization in the presence of an emulsifieror a mixture of different emulsifiers selected from the group consistingof alkali metal salts of alkyl- or alkylarylsulfonic acids, alkylsulfates, fatty alcohol sulfonates, salts of higher fatty acids with 10to 30 carbon atoms, sulfosuccinates, ether sulfonates and resin soaps,and a polymerization initiator, which comprises 1) initially introducinga mixture M1 comprising, based on M1, M1 a) from 20 to 100% by weight ofthe water needed to prepare the emulsion (emulsion water), and M1 b)from 0.1 to 100% by weight of the emulsifier, 2) subsequently adding amixture M2 comprising, based on M2, M2 a) from 90 to 100% by weight ofone or more monomers selected from styrene, α-methylstyrene, butadiene,n-butyl acrylate, methyl methacrylate and acrylonitrile, and M2 b) from0 to 10% by weight of one or more copolymerizable monomers, the monomersM2 a) and M2 b) being added alternatively in monomeric form withsubsequent in situ polymerization to give a seed latex, or as a mixtureof polymerized and monomeric form, and the seed latex polymer having aweight-average molecular mass of more than 20,000, 3) then starting thepolymerization of the resulting mixture in the presence of thepolymerization initiator at temperatures of from 5 to 95° C., 4)simultaneously or subsequently adding a mixture M3 comprising M3 a) from0 to 100% by weight, based on B, of the comonomers B, and M3 b) from 0to 25% by weight, based on A, of the diene monomers A, 5) simultaneouslyor subsequently metering in a mixture M4 comprising M4 a) the remaining75 to 100% by weight, based on A, of the diene monomers A, and M4 b) theremaining 0 to 100% by weight, based on B, of the comonomers B, andcarrying out polymerization, and 6) subsequently ending thepolymerization at a conversion above 90 and below 95%, based on the sumof the monomers, the remaining 0 to 80% by weight of the emulsion waterand the remaining 0 to 99.9% by weight of the emulsifier being addedindividually or separately from one another in one or more of steps 2)to 5).
 2. A process as claimed in claim 1, wherein said emulsifierscomprise salts of higher fatty acids with 10 to 30 carbon atoms.
 3. Aprocess as claimed in claim 1, wherein said comonomers B comprisestyrene, α-methylstyrene, p-methylstyrene, t-butylstyrene,acrylonitrile, methyl methacrylate, n-butyl acrylate or mixturesthereof.
 4. A process as claimed in claim 1, wherein the nature, amountand metering of the emulsifier are chosen such that the weight-averageparticle size d₅₀ is from 100 to 400 nm.
 5. A process as claimed inclaim 1, wherein the entirety of the comonomers B is added in step 4).6. A process as claimed in claim 1, wherein the entirety of the dienemonomers A is added in step 5).
 7. A process as claimed in claim 1,wherein the seed latex is composed of styrene as monomer M2 a).
 8. Aprocess as claimed in claim 1, wherein the weight-average particle sized₅₀ of the seed latex particles is from 10 to 100 nm.
 9. A process asclaimed in claim 1, wherein the metering rate of the monomers A and B instep 5) is chosen so that at no time during the polymerization reactionof A and B is more than 50% by weight of the total amount of monomericA+B present in the polymerization reactor.
 10. A process as claimed inclaim 1, wherein the metering rate of the monomers A and B in step 5) ischosen so that up to a conversion of 50%, based on the sum of A+B, lessthan 30%, based on the sum of unpolymerized and polymerized monomersA+B, of unpolymerized monomers are present in the polymerizationreactor.
 11. Rubber particles K prepared by a process as claimed inclaim
 1. 12. The method of using rubber particles K prepared as claimedin claim 1 as a constituent of thermoplastic molding compounds,dispersions, paper coating compositions or surface coatings or fortextile finishing.